Optical cable with filling compound and parallel fibers

ABSTRACT

Single mode optical fiber cable with a plurality of buffer tubes wherein the buffer tubes each encase a plurality of single mode optical fibers, the fibers run in a random pattern through the buffer tube and the remaining space is filled with soft filler material which prevents the ingress of moisture while allowing free movement of the fibers within the tube, the fibers all having equal overlength.

This is a division of application Ser. No. 687,601, filed Dec. 31, 1984.

BACKGROUND OF THE INVENTION

1. Field of The Invention

The invention relates to optical fiber transmission cables and inparticular to a new optical fiber cable and method and apparatus formaking it.

2. Background Information

Optical fiber cables for transmission signals are known in the art. Anoptical fiber cable may be made up of a plurality of optical fibershoused within a sheath. An optical fiber is an elongated element ofnarrow diameter and great or indefinite length. The optical fiber isformed of glass or synthetic material and comprises a core ofpredetermined refractive index and a covering of smaller refractiveindex.

Use of optical fibers entails some difficulties, in that, large numbersof fibers must be contained in a single cable. The fibers must be placedin the cable such that each fiber is free from excessive mechanicalstress which causes microbending in the fiber and a resultant increasein attenuation.

A preferred fiber optic cable utilizes fibers which propagate only onemode of light compared with multimode fibers which propagate severalmodes along the fiber core. Present single mode fibers are moresensitive to microbending losses, particularly at the longer wavelengthsused in today's systems than multimode fibers previously used fortelecommunications. The single mode fibers have inherent attenuationthat is only a fraction of that in multimode fibers available a fewyears ago. Typically, telecommunication grade single mode fibers have anattenuation of 0.3 to 0.5 db/km at 1300 nm wavelengths as compared tomultimode fibers which have attenuations of 3 to 4 db/km. Microbendingcan cause increased losses at this wavelength in the order of 0.1 to 0.3db/km. Therefore any additional attenuation due to microbending is muchmore significant with signle mode fibers. Additionally, new transmissionsystems operate at 1550 nm on single mode fibers. Microbending at thiswavelength can be in the order of 0.1 to 3.0 db/km which essentiallycould make the cable unusable for transmission at this wavelength.Therefore, reduced levels of stress and microbend induced attenuationare necessary for single mode cable designs.

In order to limit the attenuation various types of cables have beendeveloped. U.S. Pat. No. 4,153,332 discloses an optical fiber cablewherein a group of tightly bundled optical fibers are housed within atube and a plurality of those tubes are wound or stranded to produce acable when covered by an outer sheath. This cable, however, issusceptible to water damage caused by water entering through a leak inthe sheath and running along the interior space of the cable sheath andthe tubes. Moreover, the fibers in the '332 cable are of unequal overalllength. This is because the effective diameter of the cable coilsdiffers depending on fiber location. That is, when the fibers are in abundle, the fibers innermost in the coil travel a loop of smaller radiusthan do the fibers outermost in the coil.

U.S. Pat. No. 4,331,379 to Oestreich et al. provides for filling thespace within the cable between a stranded fiber bundle and the innercable wall with a soft filler. The filler improves the prevention ofmoisture ingress in the cable. However, the stranded inner bundle ofoptical fibers increases the bending stresses in the fibers andtherefore increases the attenuation.

In manufacture, the stranded bundle is easier to handle, however, moremachinery, and therefore increased cost is necessary to strand thefibers. If parallel fibers are used, they are often difficult to handleand the fibers may end up with widely divergent lengths and thereforeinconsistent transmission quality.

SUMMARY OF THE INVENTION

The invention relates to an optical fiber buffering tube which employstwo or more coated single mode optical fibers, fibers placed in a singleoversize buffer tube. The fibers are laid substantially parallel withthe tube axis and each other. The tube is simultaneously filled with asoft thixotropic gel or thickened oil. The gel or oil prevents theingress of moisture into the tube while allowing the fibers to movefreely and independently of each other. The tube is made of hard plasticmaterial which provides structural strength and protection to thefibers.

The generally parallel arrangement of the fibers results in lowerbending stresses than if the fibers were stranded into a bundle withinthe tube. The filling material provides a freedom of movement of thefibers within the tube so that the fibers remain relatively free of anynet compression or tensile stress during elongation, contraction orbending of the tube. By providing that the fibers are of substantiallyequal length, the fibers are caused to react similarly to a bend in thetube. That is, they take a position which causes the least stress andtherefore the least attenuation.

The apparatus and method of the invention provide for the manufacture ofthe fiber tube with generally parallel fiber of equal length disposedwithin it. The insertion of the fibers is accomplished by means of aseries of payoff bobbins and a controlling capstan. The controllingcapstan regulates the overlength of the fibers within the tube such thateach fiber has the same overlength. The rotational speed of thecapstan.is dependent upon the linear speed of the tube and controlled bycontrol unit.

A feature of the apparatus is its provision for a guide mechanism toprevent overlapping the fibers. This feature and the precise tolerancesof the capstan provide for uniform overlength of the fibers bypreventing overlapping of the fibers on the capstan which causesdifferences in overlength for the inner and outer fibers.

The fiber payoffs or bobbins are designed to provide uniform tension inthe various fibers. Unequal tension would create non-uniform elongationand therefore unequal fiber overlength in the buffer tube.

The optical fibers are grouped in a random pattern by funnel-like meansand fed into an extrusion aparatus.

The buffer tube is extruded around the optical fibers, with sufficientspace to allow the fibers to move freely, by the extrusion apparatus.The apparatus also fills the buffer tube with a protective filler.

Several buffer tubes may be stranded together with a back twist around acentral support member which provides compression and tensile strength.The stranding of the buffer tubes provides essentially equal pathlengths for each of the fibers, regardless of its relative positionwithin the tube. The helix formed by the stranded tube provides anelongation quality to the cable without stressing the fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the drawings wherein:

FIG. 1 is a transverse cross-sectional view of the buffer tube assemblyof the invention;

FIG. 2 is a longitudinal cross-section of the buffer tube assembly;

FIG. 3 is a side elevational view of the apparatus of the invention;

FIG. 4 is a front view of the guidewheel of the apparatus of theinvention;

FIG. 4A is an enlarged view of a portion of the guidewheel; and

FIG. 5 is a view of the completed cable assembly.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

The description is given with reference to the drawings. The same orsimilar numbers are used to denote the same or similar items throughoutthe figures.

The buffer tube depicted in FIGS. 1 and 2 is comprised of an outercovering 1 which is filled with soft filling material 2, such asthixotropic filling compound. Disposed within the tube are at least twocoated single mode optical fibers which are loosely spaced with respectto each other. The loose spacing of the fibers assures that the fibersmove freely and maintain their uniform length. The internalcross-sectional area of the buffer tube 1 is larger than thecross-sectional area of the fibers; this allows the fibers room to movewithin the buffer tube 1.

The fibers are placed in the buffer tube with an overlength. That is,each fiber has a length which is incrementally greater than the tubewhich encases it. This requires the fiber to follow a wavey path withinthe tube as long as the tubes are straight. Once the tubes are strandedinto a cable, the overlength causes the fibers to follow the generalshape of the buffer tube 1.

FIG. 3 depicts the apparatus for manufacturing the buffer tube. Thepayoff stand 4 hold a plurality of payoff reels 5 which provide equaland consistent tension to the optical fibers. Consistent tension isnecessary to provide the fibers with equal elongation so that theuniform overall length may be maintained. The fibers 3 pass between acapstan 6 and a mechanism for biasing the fibers against the capstan 6.In this embodiment the mechanism is a belt 7. This biasing mechanismcauses friction between the fibers 3 and the capstan 6. This assuresthat all the fibers 3 and the capstan 6 are traveling at uniform speed.

An important feature of the device is the guidewheel 8. The guidewheel 8keeps the fibers sufficiently separated so that overlapping of thefibers 3 does not occur along their path length. The configuration ofthis wheel is best shown in FIGS. 4 and 4A. The guiding devicescomprises a series of grooves 9 which receive the fibers 3 and separatethem in a horizontal plane. This separation prevents the fibers frombunching on the capstan 6 by putting them in a parallel but separatedrelationship.

An extruder 10 (FIG. 3), which has basic construction which is known tothose skilled in the art, is used to process resin to form the buffertube 1. A crosshead 11 allows both injection of the filling material 2and the disposition of the optical fibers 3 within the extruded buffertube 1. The fibers are gathered into a random pattern by funnel-likemeans 11A and carried through the extruder During this operation, thefibers are carried by the viscous filling material. The speed of thecapstan 6 is dependent upon the speed of the buffer tube 1 issuing fromthe crosshead 11 and is controlled by control means 6A. This assuresuniformity of overlength of each fiber along the length of the buffertube assembly.

The buffer tube 1 may then be gathered onto a spool 16 or stranded intoa cable 12 shown in FIG. 5. A strength member 13 provides axialcompression and axial tensile strength. The coating 14 providesprotection by buffering the contact between strength member 13 and theplurality of buffer tubes 1. The coating 14 also provides sufficientcircumferential space for the proper placement of the buffer tubes 1,depending on the number of buffer tubes 1 stranded about the strengthmember 13. The tubes 1 which contain the fibers 3 are stranded with afinite pitch which results in additional elongation and contractioncapability for the assembly. These additional capabilities are providedwithout inducing additional stress in the fibers 3. The stranding alsoprovides equal path lengths of the tubes regardless of their positionwithin the assembled cable 12 which reduces fiber bending stress. Theinterstices between the tubes 1 may or may not be filled with a materialto prevent the ingress of moisture. A sheath 15 is applied to the cableassembly 12 to provide extra protection.

We claim:
 1. An apparatus for making buffer tubes for optical fibercable members with optical fibers of equal length comprising:(a) payoffreels for holding spools of single mode optical fibers; (b) a capstan;(c) biasing means to bias the fibers against the capstan for pulling thefibers from the spools; (d) a guide means for directing the opticalfibers in spaced but parallel relationship as they are pulled over thecapstan; (e) a crosshead for extruding a buffer tube around the opticalfibers such that said fibers take a random path with respect to eachother; (f) means for collecting the fibers in a random groupingimmediately prior to said crosshead; and (g) means to inject soft fillermaterial within the buffer tube and fill the space within said buffertube.
 2. The apparatus according to claim 1 wherein the guide means is aguidewheel having a plurality of circumferential grooves for receivingand spacing a plurality of optical fibers.
 3. An apparatus for makingbuffer tubes for optical fiber cable members with optical fibers ofequal length comprising:(a) payoff reels for holding spools of singlemode optical fibers; (b) a capstan for pulling said fibers from saidpayoff spools; (c) guide means immediately preceding said capstan todirect said optical fibers in spaced, planar relation toward saidcapstan; (d) biasing means adjacent said capstan to hold said opticalfibers in spaced, parallel, frictional contact with said capstan as saidoptical fibers are pulled over said capstan; (e) grouping means to groupsaid optical fibers in a random pattern after they pass over saidcapstan; (f) means to extrude a buffer tube which encases said opticalfibers after said optical fibers are grouped by said grouping means; and(g) means to fill said tube with soft filler material.